1. Field of the Invention
The present invention relates to method and apparatus for determining the location of a break in a multi-pair cable. Apparatus and method in accordance with the present invention have been found to be especially useful in locating breaks in cables which are used in seismic exploration operations.
2. Description of the Prior Art
In seismic exploration, sound waves are imparted into the earth's crust at a point at or near the earth's surface. Portions of the waves are reflected from subsurface acoustic impedance boundaries, and are sensed by detectors which are arranged in arrays at the surface. The detectors may, for example, be geophones, which convert the reflected seismic waves into electrical signals.
The output of each array constitutes a "channel" of information, which is fed to a recording truck via a pair of wires. Since there are many arrays of detectors in a typical seismic system, there are many pairs of wires in the cable interconnecting the arrays and recording truck. The seismic cables are quite long and contain a plurality of sections. Each section may, for example, be 700 feet in length, and it is not uncommon for a seismic cable to include as many as 48 sections.
Although a seismic cable is farily rugged, it is common knowledge in the industry that the cable is the most vulnerable element in the system. The cable is not only subjected to a wide variety of environmental conditions, but also is flexed, thrown about and even driven over in typical day-to-day operations. A break in one or more of the cable pairs is, therefore, a common daily occurrence which is encountered by seismic crews.
The existance of a break in one of the pairs in the cable may be determined by well-known techniques for testing continuity. Once the existence of a break is determined, it is necessary to determine the location of the break, and, in the case of seismic cable, to replace the section of cable containing the break.
One approach to determine the location of a cable break has been to use a time-domain reflectometer (TDR), which is a device that transmits a pulse of energy down a cable pair. A break in the cable pair constitutes a discontinuity, which causes a portion of the transmitted pulse to be reflected. The duration of the transmitted pulse is sufficiently shorter than the propagation time to the break location so that the reflection returned from the discontinuity is easily distinguished from the original pulse. The location of the break is determined by multiplying one half the time required for the pulse to go down the line and return by a known propagation constant.
TDR has been found to be unsuitable for use with a seismic cable due to the large number of sections in the cable and since the characteristic impedance of each section is slightly different. At each connector joining two sections and with each impedance change, there will be reflections from signals traveling down the line as well as from those returning. Since seismic cable has very bad dispersive properties, the frequency components of the pulse begin separating very quickly, making it very difficult to determine the time at which the pulse returns. Attenuation characteristics diminish the magnitude of the pulse, which further aggravates the problem.
Line impedance measuring instruments have been used to determine a cable break location. These devices are usually operated at low frequencies, and as a principle of operation, assume lines short enough so the line parameters can be considered lumped and the inductive effects of the cable are ignored. This assumption results in the primary line impedance parameter being the shunt capacitance between a twisted pair of wires. By forming the ratio of the value of broken line capacitance to the value of an unbroken line reference capacitance and by multiplying that ratio by the total cable length, the break location is determined. Although capacitance can usually be measured quite accurately, the line impedance measuring method has been found to produce an error of about 20%. With a 48-section seismic cable, the location of the break could be determined within approximately ten sections. This technique has too much error to be useful in seismic applications.